In digital printers which create images from spots of light (pixels), it is important to have stable and reproducible spots. Variations in the pixel size can create many undesirable image artifacts. In certain types of printers, gas lasers are used as the light source. Gas lasers produce very predictable beam sizes, and the pixel size is almost independent of the laser source itself.
In recent years, there have been attempts to use diode lasers instead of gas lasers in laser printers. The use of diode lasers reduces the cost of the printer and permits a drastic reduction in the size and complexity of the printer. Diode lasers can be modulated directly at frequencies as high as several hundred Mhz by simply modulating the drive current, and thus, no external modulators are required. One problem, however, in using diode lasers in printer applications is the variation in beam divergence from laser to laser. This variation in the beam divergence causes a variation in the final pixel size which is approximately proportional to the divergence variation. In some types of diode lasers, the beam divergence can vary by as much as a factor of two. Thus, in order to replace gas lasers with diode lasers in laser printers, some method is generally needed to overcome the beam divergence variation of the diode lasers.
Variations in beam divergence from laser to laser can be overcome with zoom optical systems which can be adjusted according to the divergence of a particular laser. This technique will produce a fixed beam size exiting the optical system, but it has the disadvantage of introducing more components into the system. These components can be complex since diode lasers generally have elliptical light output profiles, and consequently, the optical system may need to compensate independently for each perpendicular section of the beam.
Another technique which has been used in some applications to solve the beam variation problem is to place an elliptical aperture in the beam at some convenient location. An aperture of this type is shown, for example, in U.S. Pat. No. 4,832,469. A disadvantage of these apertures is that they introduce side lobes of intensity in the beam. The light beam exiting the aperture also takes on a non-Gaussian distribution, and this makes it more difficult to compute the final spot size. Another disadvantage is that, if close control of the beam size is required, several apertures sized in small steps will be needed to cover the range of divergences found in diode lasers.
It is known in the art to use so-called apodizing filters formed on a photographic medium to control various aspects of a light beam as shown, for example, in U.S. Pat. No. 4,594,507, and U.S. Pat. No. 4,469,407. However, neither of these patents is directed to solving the problem of variation in the divergence of a beam from a diode laser.